Dual chamber pacemakers have had an enormous impact on the treatment of individuals with a variety of cardiac conditions. A dual chamber pacemaker capable of operating in a DDD mode can both pace and sense in each of the heart's chambers i.e., in the atrium and in the ventricle. Such a DDD-type dual chamber pacemaker generally requires two leads, an atrial lead for sensing and pacing in the atrium and a ventricular lead for sensing and pacing in the ventricle. The leads connect sense signals back to the pacemaker, and connect stimulus pulses from the pacemaker to the cardiac chamber. Dual chamber pacemakers can also be designed or programmed to operate in a VDD mode, wherein depolarization signals are sensed in both the atrium and the ventricle, but pacing is performed only in the ventricle. Both DDD and VDD modes enable synchronous pacing, i.e., pacing the ventricle in a timed relationship to a sensed atrial signal so as to mimic the heart's natural AV conduction, and thus provide delivery of the ventricular pace pulse at about the time that the ventricle has filled with blood due to the contraction of the atrium. Such synchronous pacing is highly desirable for maximizing cardiac output. Without it, hemodynamic efficiency may be compromised, and other untoward effects may occur.
In both DDD and VDD modes of operation, reliable atrial sensing is critical for maintenance of AV-synchrony and for other desired functions, such as mode switching. A problem that may compromise reliable atrial sensing is that of atrial undersensing, i.e., failure to sense an atrial depolarization signal. Atrial undersensing may arise from a number of different sources, including the patient's activity level, atrial lead dislodgment, improper atrial sensitivity setting, or variations in P-wave amplitude induced by respiration. Of particular importance is placement of the atrial electrode (for unipolar systems) or electrodes (for bipolar systems) with respect to the atrial wall. For 2-lead systems where a separate atrial lead is utilized, a good chronic fixation is generally achieved between the distal tip of the lead and the heart wall, although it is potentially subject to lead dislodgment. With a VDD system, a single pass lead may be used, which has one or two "floating" electrodes positioned on a portion of the lead that is in the atrium. Such an electrode or electrodes are not fixed to the atrial wall and thus are not as efficient in picking up the P-waves. While many techniques have been incorporated into dual chamber pacemaker technology for reliable atrial sensing, the occurrence of undersensing remains a problem, as failure to detect an atrial sense compromises pacemaker response.
It has long been desired in the pacemaker art to implement a pacemaker feature that provides accurate detection of atrial undersensing and that can enable either automatic or programmed adjustments to improve sensing and AV synchrony. One response in the pacemaker industry has been to test for atrial undersensing in a clinical situation, such as during pacemaker follow-up or during trans-telephonic monitoring sessions. In these situations, the clinician observes the ECG and watches for paces that do not synchronize correctly to observed intrinsic cardiac events. However, such observations are limited and may not provide the physician with sufficient information in order to reliably determine whether or not there are episodes of undersensing. Also, such clinical follow-ups are relatively infrequent, and do not provide much opportunity for relatively rapid and efficient response to undersensing conditions.
In contrast to the clinical approach for detecting atrial undersensing, applicant's invention is directed to a new approach for detecting undersensing. It includes a description of a new pacemaker feature and method for operating the pacemaker or other implanted system to enable determination of atrial undersensing based on ongoing monitoring of the patient's atrial rate, or A--A interval. In response to determined undersensing, the pacemaker may store diagnostic information that can be downloaded into an external device for the physician, or may automatically carry on synchronized pacing based on the assumption of undersensing.